Power generators for motor vehicles are operative to supply electric power to a buttery for charging the buttery and to various electrical loads for allowing the electrical loads to operate. While such a power generator is operating to supply electric power to a battery and/or various electrical loads, the disconnection of the battery from the power generator, which is referred to as “load dump”, causes a high voltage (a giant pulse) across a corresponding phase stator winding due to its large impedance. The peak of such a high voltage may get to be equal to or higher than 100 V depending on an output current of the power generator. Because this kind of a high voltage would damage any electrical loads and/or electrical elements in the power generator, some measures to reduce a high voltage have been taken.
A power generator disclosed in U.S. Pat. No. 5,748,463 corresponding to Japanese Patent Application Publication No. H09(1997)-219938 shows one of these measures. The power generator disclosed in the Patent Publication is provided with MOS transistors as low-side switching elements of a bridge rectifier thereof. The power generator is designed to, when the output voltage of the power generator exceeds a preset reference voltage, operate in protection mode to turn on at least one lower-side MOS transistor corresponding to at least one phase stator winding across which a high voltage due to load dump is generated, thus circulating current based on the high voltage through the bridge rectifier and the stator windings to decay the high voltage.
When the output voltage of the power generator becomes equal to or lower than the reference voltage based on the turn-on of the at least one low-side MOS transistor, the power generator is designed to turn off the at least one lower-side MOS transistor, and to shift to rectifying mode to carryout rectifying operations.
In addition, a power generator disclosed in Japanese Patent Application Publication No. 2003-244864 shows another of these measures. The power generator is designed to, when a high voltage is detected, shift to protection mode to drive each of high-side MOS FETs and low-side MOSFETs of a bridge rectifier in an on-off phase opposite to a normal on-off phase in rectifying mode. When the high voltage decays, the power generator is designed to shift to the rectifying mode to switch the on-off phase of each of the high- and low-side MOSFETs to the normal on-off phase in the rectifying mode.